Increased performance of circuit devices on a substrate (e.g., integrated circuit (IC) transistors, resistors, capacitors, etc. on a semiconductor (e.g., silicon) substrate) is usually a major factor considered during design, manufacture, and operation of those devices. For example, during design and manufacture or forming of, metal oxide semiconductor (MOS) transistor semiconductor devices, such as those used in a complementary metal oxide semiconductor (CMOS), it is often desired to increase performance by increasing movement of electrons (e.g., charge carriers) in N-type MOS device (NMOS) channels and/or by increasing movement of positive charged holes (e.g., charge carriers) in P-type MOS device (PMOS) channels. Increased or higher charge carrier mobility may lead to increased drive current (such as at drive current saturation), which also increases performance.
In some circuit devices, a substrate of varying layers having different crystal orientations is desired for increased performance. “Crystal orientation” refers to the crystal lattice structure of materials, such as semiconductors, used in the fabrication of semiconductor substrates. Crystal orientation planes of silicon are typically represented as <100>, <110> and <111> and are representatively shown in FIG. 1. Monocrystalline silicon is an anisotropic material, meaning that the properties of monocrystalline silicon change depending on the direction from which they are measured within the crystal lattice of silicon. This may be explained by the different atomic densities within each of the <100>, <110>, and <111> crystal planes that are illustrated in FIG. 1. The atomic densities of the <100> crystal plane, the <110> crystal plane, and the <111> crystal plane are illustrated in FIG. 1. Examples of properties that change with the direction in silicon include the Young's Modulus (a measure of the strength of the material), the mobility of electrons (or holes), the etch rate, and the oxidation rate. For example, the Young's modulus of silicon is 1.3 e12 dynes/cm2 in the <100> crystal plane, 1.7 e12 dynes/cm2 in the <110> crystal plane, and 1.9 e12 dynes/cm2 in the <111> crystal plane. As another example, the mobility of electrons in the <100> crystal plane is known to be greater than in the <110> crystal plane of silicon, resulting in a current drivability in the <100> direction that is approximately 15 percent (%) greater than the current drivability in the <110> direction.
Many integrated circuits, such as microproccesors, make use of N- and P-MOS transistors formed on the same substrate. NMOS transistors have increased charge carrier mobility and/or drive current (e.g., have increase performance or function better) on a substrate with a <100> crystal orientation. PMOS transistors, in contrast, have increased charge carrier mobility and/or drive current (e.g., have increase performance or function better) on a substrate with a <110> crystal orientation.